Differential heating apparatus for metal ingots and blocks



Dec. 18, 1951 T. A. LEDIN 2,578,890

DIFFERENTIAL HEATING APPARATUS FOR METAL INGOTS AND BLOCKS Filed April 28, 1949 d- T ig- H F X rs IL 15 c p C TIME 7'IME ---|2 K w- 'L H I l T K v I l U U \JIU \J UIU v J L n Inga/Liar? QY/icqdmp Patented Dec. 18, 1951 DIFFERENTIAL HEATING-APPARATUS FOB METAL INGOTS AND BLOCKS Theodore A. Ledin, Chicago, 111., assignor to A. Finkl & Sons 00., Chicago, lll., a corporation of Illinois Application April 28, 1949, Serial No. 90,146

-7 Claims. (Cl. 73-341) This invention relates to improvements in apparatus for controlling the temperature of metal forgings and castings while being heated. The apparatus of this invention is particularly adapted for hot-working or for the development of physical properties of steel ingots, blocks or forgings, but is not necessarily limited to such use. I

Heating of metals is commonly carried out in a suitable furnace supplied with temperature registering devices, such as pyrometers. Accurate and dependable pyrometers for automatic temperature control and recording are now commonly employed in most furnaces.

In the heating of heavy masses of steel such as ingots, blocks or the like, it is necessary that the initial heating from atmospheric temperatures to about BOO-800 F. be slow. This is to prevent breakage from thermal shock. Certain steels are more senstiive than others, and the rate of heating must be controlled accordingly. However. after this initial heating has been accomplished, the rate of subsequent heating in general need only be limited by the capacity of the furnace to produce the heat necessary to raise the steel to the required temperature. The problem then resolves itself to controlling conditions so that the piece is heated to the required temperature with as much speed as possible without danger of overheating.

Heretofore, in the interest of safety and accuracy, this has usually been done by heating the furnace to the required temperature and then holding the furnace at that temperature for a length of time sufficient for the steel to reach the required temperature. When the piece is massive and heavy, however, this is a very timeconsuming procedure for reasons that will hereinafter be more fully described in detail.

In carrying out my invention, I utilize an improved apparatus for temperature measurement, whereby the surface temperature of the piece being heated becomes the factor which controls the heating cycle.

My invention may best be understood by referringto the accompanying drawings in which:

Figure 1 is a graph showing a heating method commonly used in which the furnace temperature is controlled by the pyrometer.

Figure 2 is a graph showing an idealized method for rapid heating of a metal piece with maximum speed, but which method involves considerable risk of overheating the piece or portions thereof. In this method, the surface temperature of the piece being heated is the controlling factor.

Figure 3 is a schematic diagram oi. a pyrometer circuit including three thermocouples connected in series especially adapted for use with my improved method of heat control. 4

Figure 4 is a. graph illustrating a practical method of heating a heavy metal piece in which the surface temperature of the piece is the controlling factor. typical of those obtainable with my method of utilizing a plurality of thermocouples as shown in Figures 3, 5 and 6.

Figure 5 is a diagrammatic view showing a furnace with a block of metal therein and illustrating a preferred arrangement of a plurality' utilized in my improved system may be set forth in connection with the graphs shown in Figures 1 and 2. The graph of Figure 1 illustrates the method commonly employed for bringing a piece of metal up to a desired temperature without danger of overshooting the temperature desired.

In this graph, the curve F, the furnace temperature, is controlled as usual by a pyrometer, and at no time exceeds the temperature T. Under these conditions, the surface temperature of the piece will follow approximately the curve S and the center temperature of the piece will follow curve C. Curve C eventually reaches temperature T at H hours. Obviously, the surface temperature S and the center temperature C reach control temperature practically simultaneously. The length of time that the furnace temperature F is held at temperature T is often very long and wasteful of furnace hours. Its duration is frequently arbitrarily determined by the rule of thumb of one hour of soaking time per inch of smallest dimensions of the piece. The rate at which the piece will absorb heat is dependent entirely upon the temperature differential X, existing between the furnace temperature and the center temperature of the piece.

Obviously, if we wish to increase the rate of heating, we must increase the differential X. Obviously also, to increas the differential, the furnace temperature must exceed the control temperature T. When this is done, there is some danger of overheating the piece, at least on the The curves of this graph are.

surface. It is therefore desirable to control furnace conditions in such a way so that the surface temperature reaches. but never exceeds, control temperature. In other words, we must use the surface temperature as the control temperature, and the more quickly we bring the surface temperature to control temperature, the more quickly will the center temperature of the piece reach control temperature, and the piece be heated uniformly throughout its mass.

This method of heating, whereby the furnace is actually heated above the control temperature but whereby the surface temperature of the piece is used for controlling purposes, may be termed differential heating.

An idealized method for rapid heating by the differential method is illustrated in the graph of Figure 2. Under these idealized conditions, the piece is preheated to prevent damage from thermal shock, then transferred to a furnace previously heated to a very high temperature, as shown in the graph. Then, using the surface temperature as the controlling factor, the surface temperature curve S rises rapidly to control temperature T due to the greatly increased temperature differential X? and is held there automatically by the control pyrometer. As the temperature of 8" rises, the temperture of rises proportionately. When 8" comes within the throttling range of the pyrometer approaching control temperature, the fuel input is gradually reduced, with consequent reduction of temperature of F." "S," having arrived at control temperature, holds constant while C and F" reach control temperature at P" practically simultaneously. In this method of heating, the time necessary to bring apiece to temperature is considerably less than when the conventional method, illustrated in Figure l, is used. This is due, of course, to the greatly increased temperature differential.

As said before, this method, as illustrated by Figure 2, is idealized. It is not practical, ordinarily, to preheat a heavy piece in one furnace, and then transfer to another furnace previously heated to an extremely high temperature. There are two reasons, among others, for this:

(1) It is diflicult and expensive to handle heavy pieces; more so, when heated.

(2) It is very detrimental to the refractory lining of a furnace to heat it to a very high temperature.

Therefore, the preferred method utilized in the apparatus, forming the subject matter of this invention and illustrated in the graph of Figure 4, is more practical.

In the improved apparatus of my invention, the piece is loaded into a cool furnace and is heated slowly to eliminate the hazards of thermal shock. Then, having arrived at a, safe preheating temperature and using the surface temperature as measured by my method for controlling purposes, the furnace is heated as rapidly as the heating capacity of the furnace will permit and the controlling pyrometer will allow. Thus, the temperatures of the furnace, surface and center, will follow the curves F, S and C, respectively, with F" and C arriving at temperature T at point P simultaneously. This method, while not as rapid possibly as Figure 2, is more rapid than Figure 1, and more economical than either. The temperature differential X, while not as great initially as in Figure 2, becomes increasingly equivalent to that in Figure 2 in the latter part of the heating cycle. Differential X in Figure 4 is 4 about double that in Figure 1 throughout the entire heating cycle.

The problem, then, becomes one of measuring the surface temperature so that safe and accurate control may be obtained.

This is more difficult than at first appears. The principle of the wire thermocouple is well known and need not be gone into in great detail here. Placing the hot junction (measuring end) of a thermocouple in point contact with the surface of the piece will not, necessarily, give an accurate reading of the surface temperature. In fact, under conditions where a differential exists be tween the furnace temperature and the surface temperature, it is not possible to measure the surface temperature accurately with a standard wire thermocouple by point contact. The reason for this is that the higher temperature conditions of the furnace will have a far greater effect on the thermocouple than will the point contact with the cooler surface of the piece. The resultant temperature reading will be a compromise of the respective effects of the iumace and surface temperatures. Any method which will overcome these difliculties, such as welding the thermocouple to the piece or shielding the couple from the furnace temperature, is apt to be very cumbersome and consequently impractical.

Referring now more particularly to the improved pyrometer control circuit and apparatus for providing the required degree of temperature control, illustrated in the graph of Figure 4, but without danger of initial overheating of the piece, I utilize a series-connected" thermocouple circuit, shown diagrammatically in Figure 3. In this figure, three thermocouples, K, L and M, are connected in series in which the polarity of one of the thermocouples, as for instance M, is arranged in opposition to that of the two thermocouples K and L, as indicated by the plus and minus signs associated with the three thermocouples. Thus, the temperature, as measured at N, will equal the algebraic summation of the temperatures measured by the three thermocouples; that is to say, N =K +LM Thus, if temperature M is greater than K or L, then N will be less than K, L or M. However, if K, L and M are all equal, then N will be equal also. Accordingly, a reading N on the pyrometer will not measure a true temperature unless K, L and M are all at the same temperature.

In carrying out my improved method of heat control by a thermocouple arrangement shown in Figure 3, I make use of this principle of opposed thermocouples by placing the thermocouples in predetermined positions in the furnace, as illustrated diagrammatically in Figure 5. If the thermocouples K and L are placed together so that both are sensitive to the temperature of the metal piece, and the third thermocouple M is placed in the furnace at a point sufficiently removed from the piece to measure the temperature of the surrounding heat effective on the piece, the resulting pyrometer reading N will then represent the summation of the temperatures at the hot junctions of thermocouples K, L and M. Thus, when M is hotter than K and L, the temperature reading at N will actually be less than the temperature measured at K and L.

This is a desirable condition because, as explained before. the temperature at K and L is not an exact measure of the surface'temperature of the piece, but is actually somewhat higher. Thus, the summation temperature N, being lower than K or L, is a more usable and possibly aka more accurate measure of the actual surface temperature and, therefore, is a more useful measure for differential heating.

As one practical arrangement for mounting the thermocouples in the proper relative positions, as above described, Figure shows diagrammatically a furnace of a well-known fuelfired type, in which the hot products of combustion enter the heatingchamber, indicated at Ill, and one or more metal pieces H are placed in the furnace for heat treatment. Two of the thermocouples, K and L, are placed in the furnace in close juxtaposition to the piece so as to be responsive to the temperature of the piece. The third thermocouple M is placed in the furnace at a point removed from the piece, where it is responsive to the heat being applied within the furnace.

In the form shown in Figures 5 and 6, the two thermocouples K and L may, for convenience, be placed in a single thermocouple protection tube l2, and the tube is extended through one wall of the furnace into direct point contact with the piece I The third thermocouple M is placed in a shorter thermocouple protection tube l3 which is extended only into the open space surrounding and substantially removed from the piece in the furnace, as shown.

It will be understood that the thermocouples, K and L, will not ordinarily be expected to respond to the exact temperature of the piece unless they and their protection tubes are substantially imbedded within the surface of the piece" or are metallicly connected thereto. Nevertheless, the resultant efiects on the two thermocouples will usually be found sufliciently accurate for most purposes if their protecting tube is bought into substantially direct proximity with the surface of the piece, as indicated diagrammatically in Figure 5.

With the thermocouples of the pyrometer circuit arranged as shown in Figure 5, the pyrometerreading N can then be utilized for either automatic or manual control of the heat applied .to the furnace in the manner previously de- .and as the difierential X becomes smaller, N

will approach the true surface temperature S. Finally, when X equals zero, and the temperatures existant at K, L and M are all the same, then N will be a true measure of all the temperature conditions in the furnace.

It will also be manifest that the same principle of a summation control can be employed with more than three thermocouples arranged in series, if desired, provided that certain of them have opposed polarity to the others, and the several thermocouples are properly disposed in the furnace to record the summation of the different temperatures of said thermocouples with respect to the relative temperatures of the furnace and the piece.

My improved thermocouple control circuit can also be used advantageously when the pyrometer is not of the automatic throttling type, as previously mentioned, but the heat input of the furnace is to be, controlled manually in accordance with the visual readings of the ,rzyrcznter N. In this case, it will be understood that the actual temperature reading of the pyrometer N, as indicated by the curve S, will not give the actual temperature readings of the furnace and piece respectively until both temperatures have reached the same temperature T.

With reference to the pyrometer circuit, it

will now be understood that the total effective potential of the one or more thermocouples of one polarity should be unequal to the total effective potential of the one or more thermocouples of opposite polarity. If more than three thermocouples are to be arranged in series in the pyrometer circuit, this difference in effective potential can be maintained by providing one more thermocouple in the group of thermocouples of like polarity which are in contact with the piece, than the number of thermocouples of opposed polarity disposed at a distancefrom the piece. Thus, a differential ratio can be maintained between the opposed potentials in the circuit as long as the temperatures effective on the thermocouples are unequal to each other.

It will be further understood that the pyrometer used with my apparatus may be of standard calibration. No special instrumentation is needed-another advantage of my invention.

My method of using the series-connected thermocouples for what may be termed summation measurement affords a practical solution to the problem of producing a measure of surface temperature. This method accomplishes the result with the simplicity that point contact affords, and with the safety and accuracy that good practice demands.

Although I have shown and described certain methods and apparatus for the purpose described, it will be understood that I do not wish to be limited to the exact apparatus and construction shown and described, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defmed in the appended claims.

I claim:

1. An apparatus for measuring surface temperature of a body under heat treatment in a furnace which comprises a, pyrometer circuit including three thermocouples connected in series with two of said thermocouples arranged in additive relation and positioned substantially in direct heat responsive relation to the surface of the body, and with one of said thermocouples connected in opposition to said two thermocouples and being disposed in said furnace in materially spaced relation to said body so as to be primarily responsive to the furnace temperature surrounding said body, whereby a resultant summation of said temperatures is recorded on the pyrometer to serve as a single control for the progressive heat input to the furnace.

2 An apparatus for measuring surface temperature of a body under heat treatment in a furnace which comprises a pyrometer circuit including at least three thermocouples connected in series with a larger number of said thermocouples connected additively and in opposed polarity to the remainder of thermocouples being one less in number and disposed in said furnace in materially spaced relation to said body so as to be primarily responsive to the furnace temperature surrounding said piece, whereby a resultant summation of said temperatures is recorded on the pyrometer, to serve as a single control for the progressive heat input for the furnace.

8. In a pyrometer circuit for measuring the temperature of a body under heat treatment in a furnace, a direct reading pyrometer having three thermocouples connected in series therewith, with two of said thermocouples arranged in opposed polarity to the third thermocouple, so that a substantially correct temperature condition is indicated on the pyrometer only when all of said thermocouples are exposed to the same temperature.

4. In a pyrometer circuit for measuring the temperature of a body under heat treatment in a furnace, a direct reading pyrometer having an uneven number of thermocouples connected in series therewith, at least two of said thermocouples being of like polarity and the remainder being of opposed polarity and one less in number than the other thermocouples, so that a substantially correct temperature condition is indicated on the pyrometer only when all of said thermocouples are exposed to the same temperature.

5. An apparatus for measuring the temperature of a body being heated within a heating zone which comprises means for thermally generating a plurality of individual voltages each corresponding in value substantially to the surface temperature of said body, means for thermally generating additional voltages, in number one less than said first-mentioned voltages, and all of said additional voltages corresponding in value substantially to the temperature of said zone, means including an electric circuit for algebraically combining said voltages with the polarities of all of said additional voltages in opposition to the polarities of a like number of said first-mentioned voltages to obtain a voltage representative of the temperature of the body.

6. An apparatus for measuring the temperature of a body being heated within a, heating Y zone which comprises thermocouple means for thermally generating at least two individual voltages each corresponding in value substantially to the surface temperature of said body, thermocouple means for thermally generating at least one additional voltage corresponding in value substantially to the temperature of said zone, there being one less voltage corresponding to the zone temperature than there are voltages corresponding to the surface temperature of the body, and means including an electric circuit connecting all of said thermocouples in series for algebraically' combining all of said voltages with the polarities of the voltages corresponding to the zone temperature arranged in opposition to the polarities of a like number of the said voltages corresponding to the surface temperature of the body to obtain a voltage representative of the temperature of the body.

7. An apparatus for measuring surface temperature of a body under heat treatment in a furnace, comprising three or more thermocouples arranged in series with certain of the thermocouples arranged in opposed polarity to other, at least two of said thermocouples of like polarity being positioned within said furnace substantially in contact with the surface of said body, the remainder of said thermocouples, and in number one less than the thermocouples positioned in contact with the surface of said body, being also positioned within the furnace, materially spaced from said body so as to be responsive primarily to furnace temperature, an indicator arranged in circuit with said thermocouples and responsive to the resultant summation of temperature on said thermocouples.

THEODORE A. LEDIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,669,510 Feam May 15, 1928 1,775,682 Martin Sept. 16, 1930 

7. AN APPARATUS FOR MEASURING SURFACE TEMPERATURE OF A BODY UNDER HEAT TREATMENT IN A FURNACE, COMPRISING THREE OR MORE THERMOCOUPLES ARRANGED IN SERIES WITH CERTAIN OF THE THERMOCOUPLES ARRANGED IN OPPOSED POLARITY TO OTHER, AT LEAST TWO OF SAID THERMOCOUPLES OF LIKE POLARITY BEING POSITIONED WITHIN SAID FURNACE SUBSTANRIALLY SPACED FROM SAID BODY SO AS TO BE RESPONSIVE THE REMAINDER OF SAID THERMOCOUPLES, AND IN NUMBER ONE LESS THAN THE THERMOXOUPLES POSITIONED IN CONTACT WITH THE SURFACE OF SAID BODY, BEING ALSO POSITIONED WITHIN THE FURNACE, MATERIALLY SPACED FROM SAID BODY SO AS TO BE RESPONSIVE PRIMARILY TO FURNACE TEMPERATURE, AN INDICATOR ARRANGED IN CIRCUIT WITH SAID THERMOCOUPLES AND RESPONSIVE TO THE RESULTANT SUMMATION OF TEMPERATURE ON SAID THERMOCOUPLES. 